This invention relates to a process for settling colloidal-type suspensions and, more particularly, to a method for coagulating mineral slimes with fly ash and a polyelectrolyte as an effective means of waste disposal for both the slimes and the fly ash.
In a number of mining industries such as copper, iron (taconite), potash, phosphate, etc., waste products from the processing present serious disposal problems. For example, in the phosphate industry processing leads to about one-third recoverable phosphate rock, one-third sand tailings and one-third fines of generally less than 150 mesh particle size. Slimes are an aqueous suspension of these ultrafine soil solids which are associated with the ore and which are put into solution during the processing. Thus, phosphate slimes from a benefication plant contain approximately 5% solids and, as such, represent 150% the volume of the processed material.
In the central area of the State of Florida, where a large portion of the U.S. phosphate mining industry exists, the problem of disposal of slimes has become a major problem. At present, the slimes are contained in ponds or impounded areas surrounded by earthen dams. There the slimes are allowed to settle by gravity. However, this process takes a number of years, usually over ten, to form a 20% solids concentration. In the meantime, the land covered by the pond is lost for any useful purpose and often covers over rapidly with a plant-algae growth. In addition the dams must be maintained continuously since dam failures result in pollution of neighboring lands and waters. Also, the fresh water taken out of use by being tied-up in the slime pools presents a problem since available fresh water is in short supply in Central Florida.
Mineral slimes exhibit colloid-like properties that are believed to be largely responsible for their poor dewatering characteristics. That is to say they comprise very fine colloid-like particles suspended in water which results largely from montmorillonite and attapulgite content. Attapulgite and montmorillonite together are known to comprise approximately one-third of the slime composition. These materials are also well known for their colloid-like behavior when exposed to water. They tend to absorb water or to link with water and form a suspended material.
A typical mineralogical and chemical analysis of phosphate slime is as follows: MINERALOGICAL COMPOSITION ______________________________________ Carbonate-fluorapatite 20-25% Ca.sub.10 (PO.sub.4,CO.sub.3).sub.6 F.sub.2.sub. -3 Quartz 30-35% SiO.sub.2 Montmorillonite 20-25% (Mg,Ca)O.Al.sub.2 O.sub.3. 5SiO.sub.2.nH.sub.2 O Attapulgite 5-10% (Mg,Al).sub.5 Si.sub.8 O.sub.22 (OH).sub.4. 4H.sub.2 O Wavellite 4-6% Al.sub.3 (OH).sub. 3 (PO.sub.4).sub. 2.5H.sub.2 O Feldspar 2-3% KAlSi.sub.3 O.sub.8 + NaAlSi.sub.3 O.sub.8 Heavy Minerals 2-3% Zircon, Garnet, Ilmenite, Rutile Dolomite 1-2% CaMg (CO.sub.3).sub.2 Miscellaneous 0-1% Kaolinite, Crandal- lite, Hydrated Fe- oxide, organic CHEMICAL ANALYSIS ______________________________________ P.sub.2 O.sub.5 9-17 SiO.sub.2 31-46 Fe.sub.2 O.sub.3 3-7 Al.sub.2 O.sub.3 6-18 CaO 14-23 MgO 1-2 CO.sub.2 Trace -1 F Trace -1 Bone Phosphate of Lime 19-37 Loss on ignition at 1000.degree.C 9-16 ______________________________________
The phosphate industry has tried conventional settling practices for years for dewatering slimes and they have been virtually ineffective. The volume of slimes resulting from thickening even after prolonged settling times occupies more volume than the total volume of matrix mined. This is thought to be due to the fact that the water molecules are linked in crystalline structure with both the attapulgite and montmorillonite thus creating greater volume.
Though the industry is well aware that impoundment was not a total solution to the problem, when phosphate mining was first undertaken on a large scale there was ample area in Florida to simply store the slimes in ponds or to pump them back into the mined areas. Over the years, efforts have been made to optimize the system and to obtain maximum storage of the slimes per unit of pond retention area. However, this method amounts to providing for virtually permanent storage of the slimes and this has led to the problems discussed above. Nearly all the slimes Florida produced from the beginning of the use of matrix desliming, have been stored in impounded areas. Throughout the history of the phosphate industry in Florida, the phosphate slimes were recognized as a waste problem and were considered as a nuisance.
Over the years, many things have been considered for deatering the slimes. Equipment such as hydrocyclones, hydroclassifiers, clarifiers, and other mechanical dewatering equipment was tried. Filtration and centrifuges were also tried. Electrophoretic and electro-osmotic methods for separating liquids from suspension is widely used for dewatering of clays, particularly in Europe. These processes were, however, found to be too expensive and have not to our knowledge been used in the industry. Freeze drying was also considered and while it showed promise, the current energy crisis makes this seem impractical.
Inasmuch as various types of micro-organisms were found to be in the slimes, the Bureau of Mines explored the use of micro-organisms for dewatering the phosphate slimes. The research indicated that under certain conditions, the solids in dilute slimes could be aggregated and collected with the growth of some molds. Cost and control appear to be problems here.
A great many other things have been tried including the use of dyes, selective oiling, flotation, acid leaching, ion exchange, ultrasonic irradiation, gamma ray irradiation, induced magnetic fields and electrical fields. In some cases partial dewatering took place and in several instances the rate of dewatering was improved. However, in most cases the processes were expensive or energy requirements were excessive.
The idea of flocculation or coagulation of slimes has also attracted particular attention because of the obvious advantages to be obtained by rapidly dewatering the slimes by mere addition of chemicals. Thus, useable water can be recovered, and the compacted solids may be subject to further processing or, at the least, will take up far less area as a land fill than the area covered by the slime pond. Most importantly, the useless, space-consuming, dangerous slime pond would be eliminated from the landscape.
For this reason, the patent literature reveals a number of ways to flocculate mineral slimes such as phosphate slimes. As examples reference is made to U.S. Pat. Nos. 3,680,698 to Liu (a polyelectrolyte such as polyacrylamide used either alone or in combination with gypsum as the flocculating agent in a first step); 3,763,041 and 3,761,239 to Cook (tailings used as the slime treatment); 3,725,265 to Legal (calcium hydroxide utilized to raise the pH and precipitate the slime solids); 3,346,463 to Goren (disclosing the use of polysaccharide flocculating agents); 3,020,231 to Colwell (using a polyelectrolyte such as polyethylene oxide); 2,988,504 to Maznek (using an organic clarification agent in combination with an alkali metal silicate); 2,660,303 to Haseman (which discloses using a starch material as the flocculating agent); and 2,381,514 to Phelps (utilizing a combination of caustic soda and sodium silicate to flocculate).
However, as noted in The Bureau of Mines Report of Investigations, RI 7816, 1973 entitled "Electrophoresis and Coagulation Studies of Some Florida Phosphate Slimes", many techniques can be used in the laboratory to dewater the slimes but none are well-engineered technologies that can be applied economically. Thus, the need exists for an effective, economical system to dewater mineral slimes.
A need also exists for efficient means of disposing of fly ash. Fly ash is the well known waste product from the combustion of pulverized coal. It is collected from power plant stacks by means of mechanical and/or electrostatic precipitators. The collected fly ash is often disposed of by dumping into ponds or other disposal sites. As such, the benefits from finding useful means of disposing of fly ash are apparent.
While fly ash has been suggested as a part of an aqueous waste treatment process, to our knowledge, it has not been used in connection with treating mineral slimes. That is, in Clark U.S. Pat. No. 3,338,828 there is disclosed the treatment of aqueous wastes such as sewage by the addition of a water-soluble inorganic coagulant, small amounts of electrostatically precipitated fly ash, and an organic polyelectrolyte coagulant aid. However, in a more recent Clark U.S. Pat. No. 3,388,060 it is stated that the process disclosed in his earlier patent was found to be "not universal and does not effect material removal of phosphate from the water". In this later improvement patent, therefore, Clark discloses the use of acid-treated fly ash along with a water-soluble inorganic coagulant and an organic polyelectrolyte coagulant aid. But use of an acid-treated fly ash requires that processing steps be performed on the fly ash prior to its being used as a flocculant aid. This, accordingly, discourages the use of fly ash in this manner as a further means of disposal of the fly ash itself. Thus, there remains a need also for effective, economical means of disposing of fly ash.